Decompression-illness, or the "bends" is a well understood and serious medical condition experienced by divers being exposed to nitrogen forming in the blood stream as the diver ascends from the increased pressure experienced at depth. The amount of nitrogen forming is a direct result of the amount of nitrogen in the air stored in a diver's tanks and breathed at depth.
It is now understood that the use of air having reduced amounts of nitrogen decreases the incidence and seriousness of the bends medical condition. First developed by the National Oceanographic and Atmospheric Association (NOAA), so called "nitrox" gases having reduced levels of nitrogen were found to reduce the bends. Ambient atmospheric air has generally a 21% oxygen content at sea level and a corresponding 79% nitrogen content. Nitrox, as developed, was primarily 32% and 36% oxygen, having correspondingly decreased levels of nitrogen.
Nitrox may be manufactured by mixing pure oxygen with ambient atmospheric air. The NOAA developed a gas blending method in an attempt to get consistent, safe mixtures of nitrox. Presently the nitrox gas blending standards have been established by the International Association of Nitrox and Technical Divers (IANTD), and are based on the United States federal regulations as specified by NOAA, Occupational Safety and Health Association (OSHA), the United States Navy and the United States Coastguard.
The popularity of nitrox gas for diving has increased and subsequently created a demand in recreational and commercial diving operations and the like to be able to provide nitrox gas in remote locations for use by divers when refilling their tanks.
Applicant is aware that in the prior art NOAA gas blending method nitrox gases are not blended consistently thoroughly so as to mix the oxygen with the ambient air homogeneously. The NOAA method employs a continuous mixing device having mixing coils which attempt to blend the gases prior to entry in the compressor. The NOAA method has the disadvantage that an existing compressor has to be replaced or heavily modified and it has been found that streams of pure oxygen may flow into the compressor unmixed.
The method and apparatus of the present invention is designed to meet IANTD safety standards, preferably in a nitrox mixture having an oxygen concentration of less than 40%, where the nitrox is blended prior to being fed into the compressor.
Thus it is an object of the present invention to provide an apparatus and method for in-line blending of ambient atmospheric air with concentrated oxygen in a pre-compression turbulent mixing chamber and monitoring the level of oxygen in the resulting homogeneously blended nitrox so that the high concentration oxygen supply may be finely regulated to provide the desired nitrox concentrations.
It is a further object of the present invention to provide a simplified nitrox blending device to provide safe operation and accurate nitrox mixtures.
Applicant is aware of U.S. Pat. No. 4,860,803 which issued to Wells on Aug. 29, 1989. Wells teaches injecting oxygen into a stream of ambient air in order to produce an oxygen enriched air mixture. The mixture is then compressed and delivered to storage or scuba cylinders for use in diving or other applications. Wells requires a source of oxygen appropriate for injection into the ambient airstream and therefore a great deal of caution is required during generation of the oxygen enriched air mixture to avoid explosions and other problems typically associated with the use of oxygen.
Applicant is also aware of U.S. Pat. No. 5,611,845 which issued to Delp on Mar. 18, 1997. Delp teaches generating oxygen enriched air by use of a permeable membrane gas separation system for separating a nitrous gas component and an oxygen enriched air component from compressed air.
In the '803 patent, Wells teaches mixing ambient air with injected oxygen in a gas mixing coil. In applicant's experience, this method of mixing is problematic in that the flow through the mixing coil remains laminar resulting in poor mixing that is not detected before the gas enters the compressor. Consequently unmixed entrained oxygen can result in combustion in the compressor especially if sufficient oxygen is injected in an attempt to obtain 40% oxygen levels in the final nitrox mix. In the '845 patent, Delp discloses producing nitrox gas by removing a portion of the nitrogen content from ambient air. The use of a vortex tube is taught for dividing the nitrogen gas component into hot and cold nitrogen gas streams.